In a typical cellular network, also referred to as a wireless communication system, a User Equipment (UE), communicates via a Radio Access Network (RAN) to one or more Core Networks (CNs).
A user equipment is a device by which a subscriber may access services offered by an operator's network and services outside the operator's network to which the operator's radio access network and core network provide access, e.g. access to the Internet. The user equipment may be any device, mobile or stationary, enabled to communicate over a radio channel in the communications network, for instance but not limited to mobile phone, smart phone, sensors, meters, vehicles, household appliances, medical appliances, media players, cameras, or any type of consumer electronic, for instance but not limited to television, radio, lighting arrangements, tablet computer, laptop or Personal Computer (PC). The user equipment may be portable, pocket storable, hand held, computer comprised, or vehicle mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as for example another user equipment or a server.
User equipments are enabled to communicate wirelessly with the communications network. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between the user equipment and a server via the radio access network and possibly one or more core networks and possibly the Internet.
The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a Base Station (BS), e.g. a Radio Base Station (RBS), which in some radio access networks is also called evolved NodeB (eNB), NodeB, B node or base station. A cell is a geographical area where radio coverage is provided by the base station at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface with the user equipments within range of the base stations.
A heterogeneous network is a network comprising multiple types of radio access technologies, architectures, transmission solutions, and base stations of varying transmission power. A high power node may also be referred to as a macro node and a low power node may be referred to as a micro or a pico node. Furthermore, a macro cell is a cell that provides radio coverage served by a high power node and a micro cell is a cell that provides radio coverage served by a low power node. A macro cell typically provides larger coverage than a micro cell. In a heterogeneous network deployment the base stations typically transmit with different power levels. This leads to imbalance problems around low power nodes since the high power node is selected as the serving cell due to a higher received signal strength in the user equipment although the pathloss to the low power node is lower. To offload the high power node and also improve the UpLink (UL) performance, a cell selection offset, also known as cell Range Expansion (RE), may be used. Uplink is defined as transmission from the user equipment to the base station. Downlink is defined as transmission from the base station to the user equipment.
When the range of the micro (low power) node is extended by the RE, the user equipments in the RE zone are heavily interfered by the macro node. This interference may be mitigated in the Third Generation Partnership Project (3GPP) release 10 using so called Almost Blank Subframes (ABS), where certain SubFrames (SF) are protected, referred to as PSF (Protected SF) meaning that the macro node is not allowed to transmit user data in those SF. In a PSF the macro node is allowed to transmits some control information and system information, but it is not allowed to transmit any user data. The ABS pattern is a 40-bit string where each bit corresponds to a subframe. This may be referenced to a parameter called MeasSubframePattern. ABS may be seen as a special case of Reduced Power Subframes (RPS), where the macro node is allowed to transmit with reduced power in the PSF. Normal SF will be referred to as non-PSF. Summarized, in 3GPP release 10, the macro node is not allowed to send any uplink grants scheduling a user equipment on the Physical Uplink Shared Channel (PUSCH), and/or uplink Transmit Power Control (TPC) commands for PUCCH/PUSCH and/or downlink assignments scheduling a user equipment on the PDSCH in a PSF. In RPS, the macro node may send uplink grants, uplink TPC commands and downlink assignments in the PSF, but with reduced power. PUCCH is short for Physical Uplink Control Channel and PDSCH is short for Physical Downlink Shared Channel.
In Long Term Evolution (LTE), the PDCCH transports downlink control information and is primarily used to transmit downlink assignments, i.e. assigns downlink resources for the base station to transmit on, scheduling a user equipment for downlink transmission on the PDSCH, and uplink grants, scheduling a user equipment for transmission on the PUSCH, i.e. assigns uplink resources for the user equipment to transmit on. A PDCCH is transmitted using an aggregation of one or several consecutive Control Channel Elements (CCEs), where a control channel element corresponds to 9 resource element groups. Each resource element group in turn comprises 4 resource elements. The PDCCH is transmitted in the first n Orthogonal Frequency Division Multiple Access (OFDM) symbols where n≦4. Link adaptation for PDCCH is achieved by selecting a discrete CCE aggregation level: 1, 2, 4 or 8 in order to achieve a certain code rate based on the Downlink Control Information (DCI) transport block size that is transmitted on the PDCCH.
The Physical Control Format Indicator Channel (PCFICH) carries information about the number of OFDM symbols used to transmit PDCCH information in a subframe. The PDCCH may be configured to transmit using OFDM symbols depending on the bandwidth, duplex configuration (FDD or TDD configuration 0-6), MBSFN configuration and antenna configuration. Duplexing refers to the way downlink and uplink data is arranged in a two-way wireless transmission. There are two types of duplexing schemes: FDD and TDD, where FDD is short for Frequency Division Duplexing and TDD is short for Time Division Duplexing. MBSFN is short for Multicast Broadcast Single Frequency Network and relates to synchronized transmission from multiple cells. MBSFN is seen as a multipath propagation by the user equipment.
The Physical Hybrid ARQ Indicator Channel (PHICH) is used to transmit hybrid-ARQ (HARQ) ACK/NACKs. Multiple PHICHs are mapped to the same set of resource elements and constitute a PHICH group which each is constituted of 3 bits.
As mentioned above, when transmitting data on the downlink, the number of OFDM symbols used to transmit the control channels may be specified in order for the receiver to know where to find control information. CFI is short for Control Format Indicator and is a parameter which defines the time span, in OFDM symbols, of the PDCCH transmission for a particular downlink subframe. The CFI is transmitted using the PCFICH. The CFI is limited to the values 1, 2, 3 or 4. For bandwidths greater than ten resource blocks, the number of OFDM symbols used to contain the downlink control information is the same as the actual CFI value. Otherwise the span of the downlink control information is CFI+1 symbols. Each subframe has a time duration of 14 time slots. In OFDM, the frequency band utilized is divided into subcarriers. This means that the information is transmitted on a time frequency grid, where each entry in the grid corresponds to one subcarrier in one time slot. An entry in the grid is referred to as a resource element. A resource block (or, more precisely resource block pair) typically (for normal cyclic prefix) comprises the resource elements in 12 consecutive subcarriers and 14 consecutive time slots.
The PCFICH, PHICH and PDCCH are all mapped to resource elements inside the downlink control region, which spans the whole bandwidth with an OFDM symbol duration corresponding to the CFI transmitted on PCFICH. The PCFICH is always mapped to the first OFDM symbol, but shifted in frequency based on the cell index. Both PHICH and PDCCH data is mapped to resource elements in all OFDM symbols in the control region in order to provide time and frequency diversity. They are also both subject to interleaving and scrambling.
A conceptual illustration of mapping of a control data resource element in the downlink using two downlink antenna ports is illustrated in FIG. 1, illustrating how the PDCCH and other control data (PCFICH and PHICH) are mapped to different resource elements scattered in frequency and time inside a control region. In FIG. 1, one subframe 101 is illustrated for a subset of the total bandwidth. A subframe is divided into resource elements 103 and a resource element group 105 comprises, as mentioned above, four resource elements 103. The control region 107 comprises, in this case, the first three OFDM symbols. The PCFICH 110 is illustrated as a box with dots, the PHICH 113 is illustrated as a box with horizontal lines and a reference symbol 115 is illustrated as a box with squares. The CCE#0 117 is illustrated, in FIG. 1, with a box with sloping lines, the CCE#1 is illustrated with a box with sloping squares and the CCE#4 is illustrated with a box with waved formed lines. The thick line in FIG. 1 represents a resource block pair.
In the PSF, when ABS is used, neither the PDSCH nor the PDCCH transmission is allowed. This means that not only the downlink performance is affected, but also the uplink since no uplink grant may be sent on the PDCCH. The uplink performance will thus be degraded to the same extent as the downlink. Similarly, for RPS the PDSCH and PDCCH may only be transmitted with reduced power which means that a user equipment located far out in the macro cell may not be reached by neither PDCCH nor PDSCH. For the uplink this may be a severe restriction for power limited user equipments that may not fully utilize the available bandwidth when they are scheduled. It is often beneficial to schedule such power limited user equipments frequently with small bandwidth allocations rather than scheduling large bandwidth allocations more infrequently. In this scenario, ABS/RPS may cause severe limitation in terms of uplink performance.